1. Field of the Invention
The present invention relates to a vestigial sideband (VSB) modulation apparatus, which is used in a transmission device and the like for digital television broadcasting, for example, and generates a VSB waveform, and a method for generating a VSB-modulated wave.
2. Description of the Related Art
In a well-known VSB modulation apparatus, an input baseband signal is converted into an I-axis data signal and a Q-axis data signal by a digital signal processing unit, and each frequency band of these I-axis data signal and Q-axis data signal is limited by a band-limiting filter. In the VSB modulation apparatus, a quadrature modulating unit quadrature-modulates the I-axis data signal and Q-axis data signal output from this band-limiting filter with a carrier wave in an intermediate-frequency (IF) band and generates a double-sideband-modulated wave. The VSB modulation apparatus frequency-converts the double-sideband-modulated wave from the IF band to an ultra-high-frequency (UHF) band by a UHF up-converter, and thereafter removes a part of the modulated wave by an analog filter to thereby reshape the double-sideband-modulated wave into a VSB-modulated wave.
The VSB modulation apparatus is strongly required to be reduced in the circuit size and the equipment cost. However, in the reshaping into the correct VSB-modulated wave, since the performance requirement for the analog filter is stringent, mounting the analog filter in the VSB modulation apparatus substantially restricts the reduction of circuit size and equipment cost. Additionally, when the baseband signals of a plurality of channels are to be transmitted, an exclusive analog filter is required for each channel.
There is an example which provides a binary digital signal to a baseband shaping filter and samples the signal in the baseband shaping filter to perform calculation, and thus, to convert the binary digital signal into a baseband signal having a real part and an imaginary part, whereby the digital signal undergoes VSB modulation without using the analog filter (for example, see Jpn. Pat. Appln. KOKAI Publication No. 10-270945). Additionally, there is an example in which a video signal to be input is converted into an amplitude-modulated signal and a phase-modulated signal to phase-modulate a carrier wave in response to the phase-modulated signal, and thus, to amplitude-modulate the phase-modulated carrier wave in response to the amplitude-modulated signal, whereby the VSB-modulated wave is obtained without using the analog filter (for example, see Jpn. Pat. Appln. KOKAI Publication No. 6-217210).
However, the above conventional examples attempt the reduction of circuit size and equipment cost by eliminating the analog filter; therefore, processing for generating the VSB-modulated wave is complicated, leading to the occurrence of error.